A semiconductor light emitting element, especially light emitting diode (LED), is used for various purposes such as full color displays, traffic signals, and lights for vehicles. These applications require high light output.
High luminous flux LEDs, which are driven by large current over several hundred mA, are watched with keen interest. The LEDs generate large amounts of heat because they are driven by large currents. Semiconductor light emitting devices alone do not radiate heat well.
A conventional package for semiconductor light emitting device which has a structure of heat radiation is shown in FIG. 1 and its associated description in the Japanese patent laid open No. 2004-22802, corresponding to U.S. Pat. No. 6,480,389.
The package for semiconductor light emitting device shown in the Japanese Laid Open patent publication has a cup shaped metal substrate. An oxidized metal is provided on the top surface. A metal solder deposited layer is provided on the bottom surface. One electrode is formed on the inner surface of the cup shaped metal substrate; the other electrode is formed on the outside surface of the metal substrate.
One end of electrode of the light emitting element is mounted on the inner surface of the metal surface. The other end of the light emitting element is wire-bonded to outer surface of the cup-shaped portion.
The cup-shaped portion is filled with an electrically insulating coolant. The electrical insulated coolant is enclosed by lens.
But in the conventional package shown in Japanese patent laid open No. 2004-22802, the bonding ball and the bonding wire protrude outward from the top surface of the cup-shaped metal substrate. This is because the bonding pad for the wire from the light emitting element is provided on the top surface of the metal substrate.
In the case a plurality of light emitting devices is arrayed and connected each other, it is hard to connect their electrodes when provided on the top surface. So, another pad for connecting them is needed.
The electrode layer, which is formed on the top surface of the metal substrate via an oxidized metal, has a high electrical resistance. This high resistance causes the conventional semiconductor light emitting device to heat up when driven by a high current (such as hundreds of mA).